Chemical modification of electrode

A large amount of research has been done on chemical modification of electrodes. The authoritative treatment of this subject can be found in Bard and Faulkner (2001). Because it is a very active area of electrochemistry, this subject is being periodically reviewed. From the sensing point of view, the motivation for electrode modification has been to introduce additional flexibility in the design of, and additional control over, the electrochemical processes taking place at the electrodes. We have seen one example of such a modification already (Section 7.3 Soukharev et al., 2004). 

Chemical modification of electrode surfaces by polymer films offers the advantages of inherent chemical and physical stability, incorporation of large numbers of electroactive sites, and relatively facile electron transport across the film. Since th% polymer films usually contain the equivalent of one to more than 10 monolayers of electroactive sites, the resulting electrochemical responses are generally larger and thus more easily observed than those of immobilized monomolecular layers. Also, the concentration of sites in the film can be as high as 5 mol/L and may influence the reactivity of the sites because their solvent and ionic environments differ considerably from dilute homogeneous solutions [9]. 

Electrode modification by the attachment of various types of biocomponents holds considerable promise as a novel approach for electrochemical (potentiometric, conductometric, and amperometric) biosensors. Potentiometric sensors based on coupled biochemical processes have already demonstrated considerable analytical success [26,27]. More recently, amperometric biosensors have received increasing attention [27,28] partially as a result of advances made in the chemical modification of electrode surfaces. Systems based on... 

Gorton and coworkers have been particularly active in this field and produced an excellent review of the methods and approaches used for the successful chemical modification of electrodes for NADH oxidation [33]. They concentrated mainly on the adsorption onto electrode surfaces of mediators which are known to oxidise NADH in solution. The resulting systems were based on phenazines [34], phenoxazines [35, 36] and pheno-thiazines [32]. To date, this approach has produced some of the most successful electrodes for NADH oxidation. However, attempts to use similar mediators attached to poly(siloxane) films at electrode surfaces have proved less successful. Kinetic analysis of the results indicates that this is because of the slow charge transfer between the redox centres within the film so that the catalytic oxidation of NADH is restricted to a thin layer nearest the electrode surface [37, 38]. This illustrates the importance of a charge transfer between mediator groups in polymer modified electrodes. 

Additives have been routinely used in corrosion catalysis and electrodeposition (3,A),flelds In which metals Interface with electrolytic solutions. Studies In these areas are part of the field of modification of metal surfaces In order to change the rates of processes occurring at the surface. In recent years there has been a good deal of work on what Is known as chemical modifications of electrodes (. While these semipermanent modifications have Involved seme sophisticated Investigations, the additive field Is largely studied by a trial and error process. The work In our laboratories has been aimed at obtaining an understanding of the role of additives In these... 

As shown in this symposium, interest in chemical modification of electrode surfaces has been extended in many directions, including the study of light-assisted redox reactions, and the use of modified electrodes in electrochromic devices (1,2). Our own studies have centered on the study of metal and metal oxide electrodes modified with very thin films of phthalocyanines (PC) and on the electrochromic reaction of n-heptyl viologen on metal oxide electrodes, and on the effect on these reactions of changing substrate chemical and physical composition (A,5). 

Bard, A.J. (1983) Chemical modification of electrodes. Journal of Chemical Education, 60 (4), 302-304. 

In the case of polymers or solid supports, intrinsic functionalities can sometimes be used as anchoring groups for the coupling with biomolecules. In other cases suitable functional groups must be introduced by substitution or activation. Several excellent reviews on the chemical modification of electrodes have covered all possibilities in this respect [250-254]. For special emphasis, modiflcation procedures are shown here that may play a role in electrochemical biosensor development on the basis of different electrode materials (Figure 14-16). 

Since the pioneering work of Updike and Hicks in developing enzyme electrodes and Lane and Hubbard in direct chemical modification of electrode surfaces, a great deal of attention has been paid to developing chemically modified electrodes. The result is that virtually every substance that is electroactive, or for which its chemical reaction can be coupled to the electrode-modifying matrix and/or electron transfer mediator, can now be detected electrocheniically. Principles, techniques, and scope of application of CMEs as sensors in analytical... 

Treelike molecules are attracting increasing attention because of their unique structure and properties. Since the first report on dendrimers has been given by F. Vogtie and co-workers [1] in 1978, several synthetic pathways to dendrimers have been developed and a number of core molecules and monomers have been used to prepare different dendrimers [2]. The surface of dendrimers can be modified with many organotransition-metal complex fragments. Ferrocenyl-based dendrimers can be used in the chemical modification of electrodes, in the construction of amperometric biosensors or as multi-electron reservoirs [3]. 

Thiols are impurities distributed among petroleum products. They cause foul odor and deterioration of additives in finished products. Some thiol compounds such as 6-mercaptopurine and 6-thioguanine are used in medicine (e.g., treatment of leukemia) while others form essentials components of biological systems (e.g., amino acids). Some thiols (e.g., cysteine) are not readily detected on bare electrodes, hence the need for chemical modification of electrodes with electroactive catalysts. The use of CME also enhances sensitivity for the detection of thiols even for those that can be determined directly on bare electrodes. Phthalo-cyanines and porphyrins have been studied extensively as electrocatalysts for the detection of thiols. Electrocatalytic oxidations of cysteine and 2-mercaptoethanol have received considerable attention over many years. The aim is to lower the... 

The reality is that surface electrode modification is needed to make the ultramicroelectrode material selective for NO. Therefore, the design of modified electrode surfaces using organized layers is very attractive and provides the ideal strategy. In the general case, the chemical modification of electrode surfaces with polyelectrolytes and metal complex-based polymer films has expanded the scope of appUcation of such designed electrodes and provided a lot of options for then-use in various experimental conditions. In addition to their electrocatalytic applications, such electrodes showed a great promise for electroanalysis. As far as this aspect is concerned, substantial improvements in selectivity, sensitivity, versatiUty and reproducibility can be achieved. 

Formation of monolayers by self-assembly at electrode surfaces has been widely used in recent years. How to bind molecules on electrode surfaces has been studied extensively among electrochemists since mid-1970s, and is called chemical modification of electrode surfaces [283-285]. More recently, chemisorbed ordered organic monolayers on gold, silver, and oxides have been studied extensively and are called sdfassemhled monolayers (SAMs) [286-289]. SAMs are... 

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